Head Gasket 0.030" versus 0.040"

[baggerdad]

Mike,

Great forum for real riding build advice based on the rpm range, type of bike and how a person actually rides their bike.

I’m running the below setup in Florida, but will be switching out the RevPerf kit for an Axtell 97” kit this fall due to issues. This may be splitting hairs, but I want to use a 0.030” versus the 0.040” head gasket. My thought for using the 0.030” gasket is that it might slightly increase the 2000-3000 rpm range TQ and help throttle response. On the other hand a 0.040" gasket allows a small margin for carbon buildup and other possible variables. I vent the heads breathers to the atmosphere.

My concern is the cranking compression. Although several calculators calculate my build to be around 186 ccp psi, I’m actually at 205 psi both cylinders cold and 210 warm.

Is it possible that 3.37 gearing, 1.4kw starter, heavy-duty battery cables and an Odyssey battery - 380 cca could contribute to the higher psi numbers? The bike starts fine now without compression releases and does not detonate.

Based on calculators the bike's static compression ratio now is 9.47:1 and corrected CR is 9.0:1. With the 0.030” the corrected compression ratio will be 9.2:1, however the static compression ratio will be 9.7:1 - is that too much for a cam with an intake close of 31 degrees? The bike has 57,000 miles on it and is ridden about 9,500 miles per year.

In 5th gear SAE dyno runs the bike now pulls 90 ft lbs TQ at 1700 rpms, climbs to 100 ft lbs at 3300 rpms and the maxes at 108 ft lbs and stays above 100 till 5000 rpms. On one hand, the 1.630” exhaust valves my not helping my under 3000 rpm TQ, but they could be the reason I don’t experience detonation.

Bore: 3.938” / Stroke: 4.000” / Rod length: 7.667” / Head gasket: 0.040” / Head gasket bore: 3.960” / Cylinder head volume: 84 cc’s / Deck height: 0.000”

This is my build: 2002 FLHRI / Revolution Performance 98" Cylinders / CP Flat Top Pistons / Baisley Super Stock Heads (84cc) (1.90” intake / 1.630” exhaust) /0.040" Head Gasket / JM20G cam (Intake close 31) / Horsepower Inc 48mm TB / S&S Single Bore Tuned Induction System / RB Racing LSR 2-1 / 3.37 final gear ratio / Barnet Scorpion clutch with 66T ring gear / Dyno Jet PCIII

JM20 Gear Drive Cam / 15 - Intake Open / 31 - Intake Close / 44 - Exhaust Open / 12 - Exhaust Close / 226 - Intake Duration / 236 - Exhaust Duration / 27 - Overlap / 98 - Intake Lobe Center / 106 - Exhaust Lobe Center / 102 - Lobe Separation Angle / 0.500 - Intake Lift / 0.500 - Exhaust Lift / 0.161 - Intake Lift TDC / 0.138 - Exhaust Lift TDC

[Mike]

A corrected pressure of 9.2 would not be a worry. Thats a VERY consevative cam for a 97/98 but most other choices would requiremore compression to bring the corrected back to 9.2-9.4. But, since corrected compression is calculated using the cam intake close time, 9.2 is actually a good number to target. So in that sense, your cam would be OK.

The higher measured/vs calculated CCP is due to the intake airs' momentum. Since it takes time to stop the incoming air from moving, the cylinder will actually overfill. A 48ci cylinder may actually be cramming 50ci of air when cranking (numers used area for example only). Most calculators only use actual cylinder volume to cal CCP and do not take this into account. A fast spinning starter/ratio can make this higher yet as the amount of momentum will depend on how fast the motor is turning over. These are some of the reasons that CCP tests can be somewhat questionable when comparing bike to bike. A bike with simply a low battery will give different results vs the same bike with a strong battery. This is why VE values can be (and often are) above 100%.

With your proposed build, we would recommend the 88 tooth starter ring gear. It has a leverage (ratio) advantage over the 66, yet still stornger than the stock 110.

Going to a .030 gasket should improve the tq curve at all points but I doubt enough to really affect throttle response. Tuning would have a greater affect on this, though increasing compression can improve it. A better approach would be to install a cam with more lift to take better advantage of the ported heads. This gets back to the CCP/momentum thing. A higher lift cam will allow more cylinder fill (valve farther off the seat at any given crank angle) and yield more tq overall (or do both the .030 gasket and increased lift, with similar timing).

Last, the tq curve shape will be more influenced by the exhaust with all other things being equal. The LSR pipe is somewhat contrary to the cam choice. A change here may give greater yields to the end goal (once defined).

With good valve seals and premium fuel, carbon buildup should not be an issue. The closer quench may actually help burn them off.

[baggerdad]

Mike, thank you for the reply. I have read most of your answers on this forum and have the utmost respect for yours and Baisley's opinions above most.

I have a 84T ring gear and 9T pinion gear that I can install. Please explain the leverage advantage a 84T has over a 66T. Do you mean it takes less battery energy to turn over?

[Mike]

Stock ring/pinion have a ratio of 10.2 (102/10), the 66/9=7.3 and the 84/10=8.4. Though the stock has the best ratio for leverage (tq) it has a slow cranking speed and weak tooth design (limited in size) which can easily shear teeth when the engine kicks back on the starter. The 66 has a poor ratio causing the starter to draw more battery energy but will spin the engine faster (if battery energy is sufficient) and the larger/deeper teeth can withstand more kickback without shear. But, if cranking speed is not achieved due to low or inadequate battery energy, the engine can kick back on the starter with even more force than stock (failing the starter clutch). The 84/10 combo turns out to be a good compromise between tooth strength, battery draw and cranking speed.